The objectives of this proposal are evaluate, in normal volunteers, the effects of administering bolus PEG-rHuMGDF (3 ug/kg) subcutaneously on marrow megakaryocytopoiesis, and the viability and function of the resulting platelet product. Two cohorts of volunteers will be studied. Each cohort will receive either PEG-rHuMGDF (3 ug/kg), or placebo in a 3:1 ratio, respectively. On day 8 and day 18, we will measure changes in marrow megakaryocytopoiesis; determine the deterations in platelet number, volume, lifespan, and turnover; evaluate the modulating effects of PEG-rHuMGDF on platelet function; and, evaluate the changes in endogenous thrombopoietin (TPO) and platelet (TPO) receptor numbers.